An apparatus is described for determining the equilibrium relative humidity (e.r.h.) of confectionery syrups and the results obtained are compared with those of other workers. The identity of e.r.h. with the thermodynamic activity is pointed out and an equation for the activity coefficient as a function of concentration is derived. The agreement between e.r.h. values calculated by means of this equation and experimental results is demonstrated and some of the advantages of the use of equations of this type are discussed, including the ability to draw significant conclusions about the nature of the solvent-solute interaction.

Liquid and vapor densities are compared with those of Young (8) in Tables I11 and IV. A comparison of vapor pressures is given in Table V. It is evident that considerable differences exist between the data of Young and those obtained in this laboratory. The disagreement in vapor pressure data is in the same direction and of the same order of magnitude as that observed in Young's data on n-pentane when compared with the work of Beattie, Douslin, and Levine ( 9 ) and the work done in this laboratory (6). It can be concluded then, that the information presented here is more reliable.Benedict, Webb, Rubin, and Friend ( 4 ) have fitted an equation of state to the data of Young (8). This equation therefore also deviates from the compressibility data presented here. In general, the pressures calculated by the equation of state are higher than the observed pressures, Table VI gives a few of the residuals from the Benedict-Webb-Rubin equation in the superheated region. The residual is defined as the calculated pressure minus the observed pressure. No attempt was made to adjust the original equation as presented by Benedict, Webb, Rubin, and Friend.HE importance of vapor-liquid equilibria for the design of T distillation columns has directed much attention to the derivation of algebraic expressions for smoothing experimental data, checking the internal consistency of the results, and predicting complete equilibrium curves from a minimum of experimental points. Unfortunately, most of the equations, including the well-known Margules and Van Laar relations, are valid only for conditions of constant temperature, whereas the great majority of distillations are carried out under conditions of constant total pressure. These semitheoretical equations based on the Gibbs-Duhem equation suffer from the further practical disadvantage that their use requires conversion of the experimental equilibrium results to activity coefficients to give a nonlinear plot and subsequent reconversion to x,y data, involving an accurate knowledge of the boiling point as a function of composition. More recently, entirely empirical linear equations have been proposed by Clark (8) which accurately relate vapor to liquid compositions for many systems a t constant pressure and do not require a knowledge of the boiling points. These equations, however, involve three and sometimes four arbitrary constants.The equation now proposed is linear, is immediately applicable to experimental equilibrium measurements at constant pressure without knowledge of the boiling point curve, contains only one arbitrary constant, and has been verified for 25 different constantpressure systems involving a wide range of chemical types. The equation does not apply to systems containing water as one component. EMPIRICAL EQUATIONSZ is a function of x and y only, defined by the equation: 20 1 BENEDICT-WEBB-RUBIN EQUATION FOR ISOPENTANE P = RTd + (BoRT-Ao-Co/T*)d2 + (bRT-a)d3 + aad6 + $ [(l+ydZ)e-Yd2] Bo = 2.56386 b = 17.1441 a X 102 = 6987.77 = 226902 y X 102 = 1188.07 A0 = 4825.36 a C...